For the past several decades, the scaling of features in integrated circuits has been a driving force behind an ever-growing semiconductor industry. Scaling to smaller and smaller features enables increased densities of functional units on the limited real estate of semiconductor chips. For example, shrinking transistor size allows for the incorporation of an increased number of memory devices on a chip, lending to the fabrication of products with increased capacity. The drive for ever-more capacity, however, is not without issue. The necessity to optimize the performance of each device becomes increasingly significant.
Maintaining mobility improvement and short channel control as microelectronic device dimensions scale past the 15 nanometer (nm) node provides a challenge in device fabrication. Nanowires used to fabricate devices provide improved short channel control. For example, silicon germanium (SixGe1-x) nanowire channel structures (where x<0.5) provide mobility enhancement at respectable Eg, which is suitable for use in many conventional products which utilize higher voltage operation. Furthermore, silicon germanium (SixGe1-x) nanowire channels (where x>0.5) provide mobility enhanced at lower Egs (suitable for low voltage products in the mobile/handheld domain, for example. Many different techniques have been attempted to fabricate and optimize nanowire-based devices. However, significant improvements are still needed in the area of nanowire devices.
In the manufacture of integrated circuit devices, multi-gate transistors, such as tri-gate transistors, have become more prevalent as device dimensions continue to scale down. In conventional processes, tri-gate transistors are generally fabricated on either bulk silicon substrates or silicon-on-insulator substrates. In some instances, bulk silicon substrates are preferred due to their lower cost and because they enable a less complicated tri-gate fabrication process. In other instances, silicon-on-insulator substrates are preferred because of the improved short-channel behavior of tri-gate transistors.
On bulk silicon substrates, the fabrication process for tri-gate transistors often encounters problems when aligning the bottom of the metal gate electrode with the source and drain extension tips at the bottom of the transistor body (i.e., the “fin”). When the tri-gate transistor is formed on a bulk substrate, proper alignment is needed for optimal gate control and to reduce short-channel effects. For instance, if the source and drain extension tips are deeper than the metal gate electrode, punch-through may occur. Alternately, if the metal gate electrode is deeper than the source and drain extension tips, the result may be an unwanted gate cap parasitics. Many different techniques have been attempted to fabricate and optimize three-dimensional devices. However, significant improvements are still needed in the area of three-dimensional semiconductor devices.